Conventionally, magnetoplumbite-type hexagonal ferrite, as represented by SrO.6Fe2O3, i.e., M-type ferrite, has been the main stream in ferrite sintered magnets. Concerning such M-type ferrite magnets, efforts into improving their performance have been pursued with main aims of bringing the ferrite grain size close to the single domain grain size, aligning the ferrite grains in a magnetic anisotropic direction, and increasing the magnet density. As a result of such efforts, the properties of M-type ferrite magnets are edging towards their upper limits, meaning that it has become difficult to expect any dramatic improvements in magnetic properties.
W-type ferrite magnets are known to be capable of exhibiting magnetic properties that are far superior to M-type ferrite magnets. W-type ferrite magnets have a saturation magnetization (4πIs) about 10% higher than M-type ferrite magnets, and have about the same level for their anisotropic magnetic field. National Publication of International Patent Application No. 2000-501893 discloses a W-type ferrite magnet which consists of a composition represented by SrO.2(FeO).n(Fe2O3), wherein n=7.2 to 7.7, and the mean grain size of the sintered body is 2 μm or less and (BH)max is 5 MGOe or more. It is described that this W-type ferrite magnet is produced by undergoing the respective steps of 1) mixing SrCO3 and Fe2O3 in a required molar ratio; 2) adding C to the raw powders; 3) calcining the mixture; 4) after calcining, adding CaO, SiO2 and C to the resulting product; 5) milling to a mean particle size of 0.06 μm or less; 6) compacting the obtained milled powders in a magnetic field; and 7) sintering in a non-oxidative atmosphere.
Further, Japanese Patent Laid-Open No. 11-251127 discloses, as a W-type ferrite magnet having a maximum energy product exceeding that of conventional M-type ferrite and having a composition different from that of the conventional art, a ferrite magnet characterized by a basic composition which has an atomic ratio represented by MO.xFeO.(y−x/2)Fe2O3 (wherein M is one or more selected from among Ba, Sr, Pb and La) wherein 1.7≦x≦2.1, 8.8≦y≦9.3.
Patent Document 1: National Publication of International Patent Application No. 2000-501893
Patent Document 2: Japanese Patent Laid-Open No. 11-251127